Airports with designated smoking areas can still expose nonsmoking travelers and employees to surprising levels of cigarette smoke.

If airports haven’t gone completely smoke-free, they limit lighting up to designated areas such as ventilated smoking rooms or restaurants and bars sprinkled throughout the terminals. But according to a new study by the Centers for Disease Control and Prevention (CDC), even those steps can expose passersby to unwanted secondhand smoke.

Just outside those smoking areas, average air-pollution levels caused by secondhand smoke are five times higher than those at airports that have banned smoking indoors. (Researchers took air-quality measurements about 3 ft. adjacent to the smoking areas.) Inside the smoking zones, pollution levels soar to 23 times higher than levels at smoke-free airports. Based on these results, the CDC says that airports refusing to ban smoking in all indoor areas continue to endanger the health of passengers, not to mention the employees who are exposed on a more regular basis. Brian King, a co-author of the study and an epidemiologist with CDC’s office on smoking and health, said in a statement describing the survey: “People who spend time in, pass by, clean or work near these rooms are at risk of exposure to secondhand smoke.” Even brief exposure to secondhand smoke, a proven carcinogen, contains chemicals that can cause immediate changes and potential harm to human respiratory and cardiovascular systems, according to the Surgeon General.

“Prohibiting smoking in all indoor areas is the only effective way to fully eliminate exposure to secondhand smoke,” Dr. Tim McAfee, director of CDC’s office on smoking and health, said in a news release.

If you’re wondering whether you’ll be passing through one of the airports that still permits smoking, the report targeted five: Hartsfield-Jackson Atlanta International Airport, Washington Dulles International Airport, McCarran International Airport in Las Vegas, Denver International Airport and Salt Lake City International Airport. Last year, approximately 15% of U.S. air travel took place at these five airports combined.

The smoke-free airports that served as a control for the air-quality evaluations were Chicago O’Hare International, Fort Lauderdale–Hollywood International, Orlando International and Phoenix Sky Harbor International. At all the airports in the study, researchers recorded average levels of particulates that passersby would breathe in — specifically the kind released by burning cigarettes.

Federal laws passed from 1988 to 2000 prohibit smoking on all U.S. and international flights, but there is no federal statute that mandates smoke-free airports, which the scientists say would be an important next step in reducing risk of cancer, heart and respiratory problems and other conditions linked to inhaling cigarette smoke. Earlier this week, the CDC reported that 60% of U.S. cities have banned smoking in all indoor areas like restaurants and offices, meaning that as many as 50% of Americans are protected from secondhand smoke’s potentially hazardous effects.